Every soundex code consists of a letter and three numbers, such as H525. The letter is always the first letter of the surname. The numbers are assigned to the remaining letters of the surname according to the soundex guide shown below. Zeroes are added at the end if necessary to produce a four-character code. Additional letters are disregarded.

Soundex Coding Guide:

Additional Soundex Coding Rules:

Names With Double Letters
If the surname has any double letters, they should be treated as one letter. For example: Gutierrez is coded G-362 (G, 3 for the T, 6 for the first R, second R ignored, 2 for the Z).

Names with Letters Side-by-Side that have the Same Soundex Code Number
If the surname has different letters side-by-side that have the same number in the soundex coding guide, they should be treated as one letter. Examples: Pfister is coded as P-236 (P, F ignored, 2 for the S, 3 for the T, 6 for the R). Jackson is coded as J-250 (J, 2 for the C, K ignored, S ignored, 5 for the N, 0 added). Tymczak is coded as T-522 (T, 5 for the M, 2 for the C, Z ignored, 2 for the K). Since the vowel "A" separates the Z and K, the K is coded.

Names with Prefixes
If a surname has a prefix, such as Van, Con, De, Di, La, or Le, code both with and without the prefix because the surname might be listed under either code. Note, however, that Mc and Mac are not considered prefixes. For example, VanDeusen might be coded two ways: V-532 (V, 5 for N, 3 for D, 2 for S) or D-250 (D, 2 for the S, 5 for the N, 0 added).

Consonant Separators
If a vowel (A, E, I, O, U) separates two consonants that have the same soundex code, the consonant to the right of the vowel is coded. Example: Tymczak is coded as T-522 (T, 5 for the M, 2 for the C, Z ignored (see "Side-by-Side" rule above), 2 for the K). Since the vowel "A" separates the Z and K, the K is coded. If "H" or "W" separate two consonants that have the same soundex code, the consonant to the right of the vowel is not coded. Example: Ashcraft is coded A-261 (A, 2 for the S, C ignored, 6 for the R, 1 for the F). It is not coded A-226.

Using the Soundex coding system it can be seen that Heneghan is therefore coded as H525

It can also be seen that other surnames will share the same Soundex code, for example:

Hannigan, Hanneken, Hansen, Hanson, Hansson, Henson, Henigan, Hennigan, Hinchman, Hinkson, Hinson, Hinzman, Honkanen